High nitrate (NO₃⁻) in a reef tank — diagnosis and fixes

DiagnosticMore than 1–2 small pinches of food per day for typical stocking. Uneaten food on the sand. Multiple fish are obese.

FixCut feeding by 30%. Use a target feeder for corals (Coralfeeder, Julian's Thing) so food goes where it's used. Soak frozen food in selcon or vitamins for nutrient density, not volume.

DiagnosticBioload is fine, feeding is reasonable, but NO3 climbs anyway. No refugium, no scrubber, no carbon dosing. Skimmer skimming dry.

FixAdd a chaeto refugium (8–20 gal sump section with reverse-photoperiod LED) or hang-on algae scrubber. Or start vinegar dosing per Randy Holmes-Farley's protocol — see /guides/vinegar-dosing-reef-tank/.

DiagnosticSkimmer makes dry foam only, never wet skimmate. Skimmer rated for less than 2× your display volume.

FixRe-tune wetter (raise collection cup, lower water level), or upgrade. Sizing guide: see /guides/sizing-a-protein-skimmer.

DiagnosticDust storms appear when you blow on the rock with a turkey baster. Brown 'snow' settles in low-flow areas.

FixAdd flow (powerheads) to dead corners. Weekly turkey-baster blow-down of rocks during water change.

DiagnosticTank is >5 years old, deep sand bed never disturbed. NO3 and PO4 both climbing despite normal husbandry.

FixConvert to bare bottom (best long-term fix) or do partial sand replacement: siphon out and replace 1/4 of sand bed at a time, 3 weeks apart.

DiagnosticTDS meter on RODI output reads >2 ppm. Or you're not measuring TDS post-RODI at all.

FixChange RODI cartridges. DI resin is exhausted at 0 → ~2 ppm TDS. Membranes (RO) last 2–3 years; sediment + carbon every 6 months.